The demand for hydrogen is ever increasing because of its use in various hydro-treating processes in the petroleum industry and also for its use in PEM fuel cells. Since hydrogen is a non-polluting fuel, its use as a fuel, particularly for fuel cells has been rapidly increasing. Hydrogen is a clean fuel in the sense that no CO2 is emitted during the electro-chemical conversion to water. Also, hydrogen produced via the direct cracking of methane is free of CO, which is a poison for Pt-containing catalysts. As a consequence, hydrogen generated via such a process can be used directly in a fuel cell.
Most of the hydrogen produced from methane is produced via the steam reforming process, which consists of three main steps: (a) synthesis gas generation, (b) water-gas shift reaction, and (c) gas purification (CO and CO2 removal). In order to attain the maximum conversion of methane, the process generally employs a steam/carbon ratio of about 3 to 5, a process temperature of about 800–900° C., and pressures from about 30 to 35 atmospheres. The total CO2 emission from steam reforming reaches up to about 0.35 to 0.42 m3 per each m3 of hydrogen produced. Furthermore, for certain applications, such as for PEM fuel cell technology, additional purification steps are necessary. In this regard, it is essential to reduce the carbon oxide concentration to only a few wppm, or less, in order to prevent poisoning of the Pt electro-catalyst.
Thermal decomposition is another method that can be used for producing substantially CO2-free hydrogen from natural gas:CH4→C+2H2 ΔH0=75.6 kJ/molThe methane decomposition reaction is a moderately endothermic process. The energy requirement per mol of hydrogen produced (37.8 kJ/mol) is considerably less than that for the steam reforming process (63.3 kJ/mol H2). In contrast to steam reforming, hydrogen produced from methane by thermal decomposition does not involve a water-gas shift reaction CO2 removal steps, which significantly simplifies the process. In addition to CO-free hydrogen as the major product, the process produces high-purity carbon as a by-product.
Thermal decomposition of natural gas has been used for decades for the production of carbon black with hydrogen being a supplementary fuel for the process (Thermal Black Process). Such a process has been practiced in a semi-continuous fashion using two tandem reactors at high operational temperatures (about 1400° C.). Attempts have been made to reduce the maximum temperature of the decomposition of methane via the use of catalysts. Data on the catalytic decomposition of methane using Co, Cr, Fe, Ni, Pt, Pd, and Rh-based catalysts have been reported in the literature, such as by M. A. Ernaakova et al., Journal of Catalysis, Volume 201, page 183, 2001, which is incorporated herein by reference. Among all the catalysts investigated, nickel appears to provide the highest activity and is most commonly used for this purpose.
A two-step process, in which the catalytic decomposition of methane was firstly carried out followed by regeneration of the deactivated catalyst with O2, CO2 or H2O, has been suggested for the generation of hydrogen by Choudhary et al., Journal of Catalysis, Volume 192, page 316, 2000. In this approach, the deposited carbon, a valuable by-product, was burned off as CO and/or CO2. Consequently, such a method does not offer any significant advantages over that of conventional steam reforming, because of the large emissions of carbon oxides. Furthermore, the catalyst needs to be reduced again prior to the next cycle and its use is limited due to the irreversible transformation of metallic particles that occurs during regeneration and reaction steps.
Direct catalyzed decomposition of methane offers two major advantages over the thermal route: (i) the operational temperature can be dramatically lowered from about 1400° C. to about 550° C., thus requiring less energy input for the process, and (ii) various catalytically-engineered carbon nano-structures of high value can also be generated by the judicious choice of catalysts, thus rendering the process more financially attractive. The large amount of natural gas available makes the catalytic decomposition of methane to produce hydrogen and high value carbon nano-structures commercially feasible.
In order for the direct catalyzed decomposition of methane to be of practical significance it is essential to have a highly effective catalyst for such a process, which heretofore has been unavailable. Such a catalyst should exhibit high activity for a prolonged period of time and continue to function in the presence of large amounts of accumulated carbon. Catalytic decomposition of methane is, however, a rather capricious process due to the exacting requirement for a critical metal particle size and the tendency of the reaction conditions to exert a detrimental influence on catalyst morphology. Previous work has shown that the highest yield of solid carbon was obtained for a catalyst where the average particle size was about 30 to 40 nm. (M. A. Ermakova et al. Catalysis Letters Vol. 62, p. 93 (1999)). XRD analysis indicated that Ni particles were undesirably aggregated as soon as the catalyst came into contact with methane. This particle-sintering behavior resulted in a lowering of the catalytic activity for methane decomposition.
While the concept of generating hydrogen from the catalyzed decomposition of methane has been shown to be a feasible route, there still remains a need for the development of catalysts that can achieve a higher level of performance than those currently available.